1. Field of the Invention
This invention relates generally to corona ignition assemblies, and methods of manufacturing the corona ignition assemblies.
2. Related Art
Corona igniter assemblies for use in corona discharge ignition systems typically include an ignition coil assembly attached to a firing end assembly as a single component. The firing end assembly includes a center electrode charged to a high radio frequency voltage potential, creating a strong radio frequency electric field in a combustion chamber. The electric field causes a portion of a mixture of fuel and air in the combustion chamber to ionize and begin dielectric breakdown, facilitating combustion of the fuel-air mixture. The electric field is preferably controlled so that the fuel-air mixture maintains dielectric properties and corona discharge occurs, also referred to as non-thermal plasma. The ionized portion of the fuel-air mixture forms a flame front which then becomes self-sustaining and combusts the remaining portion of the fuel-air mixture. The electric field is also preferably controlled so that the fuel-air mixture does not lose all dielectric properties, which would create thermal plasma and an electric arc between the electrode and grounded cylinder walls, piston, or other portion of the igniter.
Ideally, the electric field is also controlled so that the corona discharge only forms at the firing end and not along other portions of the corona igniter assembly. However, such control is oftentimes difficult to achieve due to air gaps located between the components of the corona igniter assembly where unwanted corona discharge tends to form. For example, although the use of multiple insulators formed of different materials provides improved efficiency, robustness, and overall performance, the metallic shielding and the different electrical properties between the insulator materials leads to an uneven electrical field and air gaps at the interfaces. The dissimilar coefficients of thermal expansion and creep between the insulator materials can also lead to air gaps at the interfaces when operating in the −40° C. to 150° C. temperature range. During use of the corona igniter, the electrical field tends to concentrate in those air gaps. The high voltage and frequency applied to the corona igniter assembly ionizes the trapped air causes unwanted corona discharge. Such corona discharge can cause material degradation and hinder the performance of the corona igniter assembly.
In addition, the different materials disposed radially across the assembly can lead to an uneven distribution of electrical field strength between those materials. While moving from the coil to the firing end, the electrical field loads and unloads the capacitance in a direction moving radially between the electrode and external shield. The electrical field concentrated at the interfaces between the different electrode and insulator materials, and in any cavities or air voids between the materials, is typically high. Oftentimes, this voltage is higher than the voltage of corona inception, which could contribute to the unwanted corona discharge along the interfaces, cavities, or air voids.